# year    value 
 1979 6.436665 
 1980 6.832164 
 1981 7.144394 
 1982 6.579129 
 1983 6.868155 
 1984  7.28403 
 1985 6.880536 
 1986 6.416978 
 1987 6.581447 
 1988 7.389573 
 1989 7.282187 
 1990  7.52317 
 1991 7.616729 
 1992 6.079296 
 1993 7.261639 
 1994 7.332922 
 1995 7.420669 
 1996 6.631419 
 1997 7.005953 
 1998 7.891675 
 1999  6.86102 
 2000 7.346686 
 2001 7.704393 
 2002 7.596728 
 2003  7.76879 
 2004 7.152732 
 2005 7.633441 
 2006 7.771615 
 2007 7.959167 
 2008 7.509151 
 2009 7.391374 
 2010 8.007402 
 2011 8.061365 
 2012 8.604845 
 2013 8.050501 
 2014 7.832446 
 2015 7.617608 
 2016 8.835059 
 2017 7.846992 
 2018 7.886228